1. Technical Field
The present invention relates generally to improved lubricating oil compositions containing additives and additive mixtures for use in internal combustion engines
2. Description of the Related Art
Automobile spark ignition and diesel engines have valve train systems, including valves, cams and rocker arms which present special lubrication concerns. It is extremely important that the lubricant, i.e., the engine oil, protects these parts from wear. It is also important for engine oils to suppress the production of deposits in the engines. Such deposits are produced from non-combustibles and incomplete combustion of hydrocarbon fuels (e.g., gasoline, diesel fuel oil) and by the deterioration of the engine oil employed.
Engine oils typically use a mineral oil or a synthetic oil as a base oil. However, simple base oils alone do not provide the necessary properties to provide adequate wear protection, deposit control, etc. required to protect internal combustion engines. Thus, base oils are formulated with various additives (for imparting auxiliary functions) such as, for example, ashless dispersants, metallic detergents (i.e., metal-containing detergents), antiwear agents, antioxidants (i.e., oxidation inhibitors), viscosity index improvers and the like to produce a compounded oil, i.e., a lubricating oil composition.
A number of such engine oil additives are known and employed in practice. The most common additive for engine lubricating oils has been zinc dialkyldithiophosphates because of their favorable characteristics as an antiwear agent and performance as an oxidation inhibitor. However, a problem has arisen with respect to the use of zinc dialkyldithiophosphate, because phosphorous and sulfur derivatives poison catalyst components of catalytic converters. This is a major concern as effective catalytic converters are needed to reduce pollution and to meet governmental regulations designed to reduce toxic gases such as, for example, hydrocarbons, carbon monoxide and nitrogen oxides, in internal combustion engine exhaust emission. Such catalytic converters generally use a combination of catalytic metals, e.g., platinum or variations, and metal oxides, and are installed in the exhaust streams, e.g., the exhaust pipes of automobiles, to convert the toxic gases to nontoxic gases. As previously mentioned, these catalyst components are poisoned by the phosphorous and sulfur components, or the phosphorous and sulfur decomposition product of the zinc dialkyldithiophosphate; and accordingly, the use of engine oils containing phosphorous and sulfur additives may substantially reduce the life and effectiveness of catalytic converters. Therefore, it would be desirable to reduce the phosphorous and sulfur content in the engine oils so as to maintain the activity and extend the life of the catalytic converter.
There is also governmental and automotive industry pressure towards reducing the phosphorous and sulfur content. For example, United States Military Standards MIL-L-46152E and the ILSAC Standards defined by the Japanese and United States Automobile Industry Association at present require the phosphorous content of engine oils to be at or below 0.10 wt. % with future phosphorous content being proposed to even lower levels, e.g., 0.08 wt. % by January, 2004 and below 0.05 wt. % by January, 2006. At present, there is no industry standard requirement for sulfur content in engine oils, but it has been proposed that the sulfur content be below 0.2 wt. % by January, 2006. Accordingly, it would be desirable to decrease the amount of zinc dialkyldithiophosphate in lubricating oils still further, thus reducing catalyst deactivation and hence increasing the life and effectiveness of catalytic converters while also meeting future industry standard proposed phosphorous and sulfur contents in the engine oil. However, simply decreasing the amount of zinc dialkyldithiophosphate presents problems because this necessarily lowers the antiwear properties and oxidation inhibition properties of the lubricating oil. Therefore, it is necessary to find a way to reduce phosphorous and sulfur content while still retaining the antiwear and oxidation or corrosion inhibiting properties of the higher phosphorous and sulfur content engine oils.
In order to compensate for lowering the amount of zinc dialkyldithiophosphate, other oxidation inhibitors such as phenol derivatives, e.g., high overbased phenates, and ashless antioxidants, e.g., alkylated diphenylamines, have been used. However, the use of such known oxidation inhibitors in place of zinc dialkyldithiophosphate at best only marginally satisfies the required levels of antiwear, oxidation inhibition and deposit control.
Detergents have been added to impart a total base number (TBN) to neutralize acidic combustion products and to clean surfaces containing deposits. However, detergents may impart undesirable properties. For example, overbased sulfonates such as magnesium sulfonate detergents are also effective to enhance the antiwear properties in valve train systems, but have drawbacks in that crystalline precipitates are sometimes produced when these engine oils are stored under humid or variable temperature conditions for a long period of time. Such precipitates may cause plugging of the filter which is installed in the engine oil circulating system. Such plugging is more likely to occur when a large amount of the magnesium sulfonate detergents is used to enhance the desired antiwear properties. Additionally, the use of high overbased detergents such as, for example, sulfonates or phenates, and low overbased sulfonates contribute toward the sulfur content which, as previously mentioned, has been proposed for significant reduction in the levels contained in the lubricating oils.
Accordingly, as demand for further decrease of the phosphorous content and a limit on the sulfur content of lubricating oils is very high, this reduction cannot be satisfied by the present measures in practice and still meet the severe antiwear and oxidation-corrosion inhibiting properties, as well as cleanliness (i.e., deposit protection) required of today's engine oils. Thus, it would be desirable to develop lubricating oils, and additives and additive packages therefore, having lower levels of phosphorous and sulfur but which still provide the needed wear, oxidation-corrosion and deposit protection now provided by lubricating oils having, for example, higher levels of zinc dialkyldithiophosphate, but which do not suffer from the disadvantages of the lubricating oils discussed above.
U.S. Pat. No. 6,278,006 discloses transesterifying triacylglycerol containing oils, e.g., vegetable oils, with short saturated fatty acid polyol esters to obtain an oil having improved lubrication properties. The patent further discloses that the oils can also contain various other additives such as, for example, antioxidants, anti-foam additives, anti-wear additives, corrosion inhibitors, dispersants and detergents.